The present invention relates to a circuit for monitoring the triggering circuit of a safety device in a motor vehicle, the triggering circuit having a triggering arrangement representing an ohmic resistance, which are connected in series with a capacitor, and a switching arrangement for charging and discharging the capacitor, and with a measuring device being present which determines the voltage drop across the triggering circuit in order to therefrom determine the resistance of the triggering arrangement and the capacitance.
Such an alternating current triggering of ignitors for airbags or seat belt tensioners is described in European Published Patent Application No. 577 988. In order to guarantee fail-safe operation of the triggering circuit of a safety device in the event of a crash, the triggering circuit must be continuously monitored. If a defect is found in the triggering circuit, this is signaled in the vehicle visually or acoustically. In practice, the following defects may occur in the triggering circuit: a short circuit may occur between the ignitor and the battery voltage or the ground. In addition, parallel connections may occur between the leads of the triggering circuit itself or parallel to the triggering circuit capacitor. According to the monitoring circuit known from European Published Patent Application 577 988, the voltage drops across the triggering circuit are measured during the different charging states of the capacitor in the triggering circuit and it is determined from the value of the voltage whether the ignitor or the capacitor is defective. International Published Patent Application No. WO 90/02673 also describes a triggering system in which a triggering circuit having a triggering arrangement and a capacitor connected in series therewith can be connected to a supply voltage via two switches. To check the resistance of the triggering arrangement and the capacitance in the triggering circuit, the capacitor is charged to different voltages in a plurality of switching cycles of the switch. The charging voltages picked up at a measuring resistor are measured, and the discharge time constant is determined. From these two quantities is determined, via comparison with reference values in a control circuit, whether or not the triggering circuit is defective.
According to the features of claim 1, the triggering means and the capacitor of the triggering circuit are arranged in a shunt arm of the transistor bridge circuit. A measured current source is connected to this transistor bridge circuit. A controller controls the transistors and a measured current source during the monitoring cycle so that the capacitor is charged by a measured current. A sample-and-hold circuit controlled by the controller detects, after the capacitor has been charged, a first voltage drop across the triggering circuit and, after the measured current has been turned off, a second voltage drop across the triggering circuit, so that the controller can determine the resistance of the triggering arrangement and the capacitance from these two voltages.
With this circuit arrangement, the measuring circuit voltages applied to the triggering circuit can be measured quickly and accurately, so that a defect in the triggering circuit can be determined in a highly reliable manner. The use of a transistor bridge has the advantage that, even in the event of a single-sided short circuit of the transistor bridge with the supply voltage, the safety device can still be ignited. The transistor bridge also has a more uniform heat load on the individual transistors. An external resistor of the triggering circuit, such as known from the related art, is not needed in the arrangement according to present invention which increases the energy transfer to the ignitor per unit of time.
Thus the measured current source includes a series circuit of a switching transistor and a measuring resistor which is connected to the shunt arm of the transistor bridge circuit, as well as to a point of the transistor bridge circuit connected to a supply voltage. A diode can be connected in series with the switching transistor for polarity reversal protection.
The controller synchronously controls the switching transistor of the measured current source and the switch of the sample-and-hold circuit. At the input of the sample-and-hold circuit there is an operational amplifier, whose output current flows via the switch, and a current switched through by the switch is supplied to an amplifier, which has a storage capacitor in a feedback branch. The output of the sample-and-hold circuit is fed back to an input of the operational amplifier. The switch of the sample-and-hold circuit can be a transmission gate transistor. The input stage of the amplifier may have one or more MOS transistors.